Recently, in a field of an oceanographic survey, a sound wave having low frequency is often used because of low attenuation and good propagation characteristic in water, and thus various low frequency transducers are realized for a practical use.
For example, Japanese Patent Application Laid-Open No. Hei-3 (1991)-11898 (patent document 1: refer to page 3, FIG. 1 and FIG. 3) shows a transducer where a metal cylindrical body is provided with a plurality of slits elongated in an axial direction thereof to form a plurality of vibrating plates, and a piezoelectric vibrator is bonded on either inside or outside of each of the vibrating plates. By using thin plate-shaped vibrating plates, a frequency of a sound wave is lowered.
Other bending vibration type acoustic transducer is disclosed in Japanese Patent Gazette No. 3520837 (patent document 2). In this bending vibration type acoustic transducer, a cylindrical vibrator is formed of laminated and combined vibrator pieces. The cylindrical vibrator includes a plurality of side-face vibrating plates (first vibrating plates for sound wave emission) and end-face vibrating plates (second vibrating plates for sound wave emission). A plurality of slits formed in an axial direction of a cylindrical metal plate form the side-face vibrating plates therein. In the bending vibration type acoustic transducer, the cylindrical vibrators vibrate in response to applied voltage signal. The side-face vibrating plates and the end-face vibrating plates perform bending vibration to generate a sound wave. Since two kinds of vibrating plates, the side-face vibrating plates and the end-face vibrating plates, are used as vibrating plates for sound wave emission, a frequency of a sound wave is lowered and a frequency band is broadened.
A transducer which radiates a sound wave in water and receives the sound wave in water uses an oscillator utilizing a structure of mechanical resonance so as to efficiently transmit the sound wave. Generally, mechanical structure tends to decrease its mechanical resonance frequency when its size is large. Thus, in order to form a transducer for low frequency, large mechanical size is inevitably needed. Accordingly, in the transducer for the low frequency, the miniaturization and the weight saving are always required, and the realization of a vibrator or oscillator which has a small size with lowered mechanical resonance is a technical issues.
Because a thin plate such as metal plate has low elasticity, its resonance frequency can be made low, thus a conventional transducer for low frequencies employs a plate-shaped vibrator which uses the thin plate as a vibrating plate so as to be a simple structure with small size and light-weight transducer. In the transducers described in the patent documents 1 and 2, the above-mentioned vibrating plate is used to lower the frequency of a sound wave.
However, the resonance frequency of the plate-shaped vibrator using the vibrating plate is determined by a length of long side and thickness thereof in the case of the rectangular plate type, or it is determined by the diameter of the disk and thickness of the vibrating plate in the case of the disk type. Accordingly, in order to aim at further lowering frequency of the transducer using the vibrating plate, thickness of the vibrating plate must be made thin theoretically or make its size large. However, in view of the strength of the whole vibrating plate, its production method is limited. Therefore, the size of the vibrating plate has to be made large. Even if the vibrating plate is used, in order to aim at further lowering its frequency, the transducer's size tends to become large and its weight is also increased, and thus causing such problems as difficulty for achieving small size and light-weight for the transducer.